The present invention relates to therapeutic radiation sources, and in particular to miniaturized, highly efficient, optically-driven therapeutic radiation sources.
In the field of medicine, therapeutic radiation such as x-ray radiation and xcex3-ray radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large, fixed position machines as well as small, transportable radiation generating probes. The current state-of-the-art treatment systems utilize computers to generate complex treatment plans.
Conventional radiation systems used for medical treatment utilize a high power remote radiation source, and direct a beam of radiation at a target area, such as a tumor inside the body of a patient. This type of treatment is referred to as teletherapy because the radiation source is located a predefined distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. Teletherapy radiation sources, which apply radiation to target regions internal to a patient from a source external to the target regions, often cause significant damage not only to the target region or tissue, but also to all surrounding tissue between the entry site, the target region, and the exit site.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to or in some cases within the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (xe2x80x9cbrachyxe2x80x9d), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of xe2x80x9cseeds,xe2x80x9d i.e. encapsulated radioactive isotopes, which can be placed directly within or adjacent the target tissue to be treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment.
The term xe2x80x9cx-ray brachytherapyxe2x80x9d is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient""s body, is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., and U.S. Pat. No. 5,422,926 to Smith et al., all owned by the assignee of the present application, all of which are hereby incorporated by reference.
The x-ray brachytherapy system disclosed in the above-referenced patents includes a miniaturized, insertable probe which is capable of generating x-ray radiation local to the target tissue, so that radiation need not pass through the patient""s skin, bone, or other tissue prior to reaching the target tissue. The insertable probe emits low power x-rays from a nominal xe2x80x9cpointxe2x80x9d source located within or adjacent to the desired region to be affected. In x-ray brachytherapy, therefore, x-rays can be applied to treat a predefined tissue volume without significantly affecting the tissue adjacent to the treated volume. Also, x-rays may be produced in predefined dose geometries disposed about a predetermined location. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy typically involves positioning the insertable probe into or adjacent to the tumor, or into the site where the tumor or a portion of the tumor was removed, to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a housing, and a hollow, tubular probe or catheter extending from the housing along an axis and having an x-ray emitting target at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optic cable enclosed within a metallic sheath. In such a flexible probe, the electron source may be a photocathode. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, the flexible fiber optic cable couples light from a laser source or a LED to the photocathode.
It is possible to reduce the power requirements of miniaturized therapeutic radiation sources used in x-ray brachytherapy, by optically driving the thermionic cathodes in the electron sources, instead of ohmically heating the thermionic cathodes. U.S. patent application Ser. No. 09/884,561 (commonly owned by the assignee of the present application and hereby incorporated by reference)(hereinafter the xe2x80x9c""561xe2x80x9d application) discloses a miniaturized therapeutic radiation source that includes a reduced-power, increased efficiency electron source that is optically driven. The ""561 application discloses an electron source that includes a thermionic cathode having an electron emissive surface. The ""561 application discloses using laser energy to heat the electron emissive surface of the thermionic cathode, instead of heating the electron emissive surface of the thermionic emitter using conventional ohmic heating. In this way, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the power requirements for the therapeutic devices. Electrons can be generated with minimal heat loss, without requiring a vacuum-fabricated photocathode.
In the devices disclosed in U.S. Pat. Nos. 5,133,900 and 5,428,658, and in the ""561 application, the electron source and the target element are enclosed within a substantially rigid capsule. The electron source generates an electron beam along a beam path, and the target element is positioned in the beam path. An accelerating electric field may be established within the capsule. The accelerating electric field acts to accelerate the electrons emitted from the electron source toward the target element. The target element emits therapeutic radiation in response to incident electrons from the electron source.
The capsule defines a substantially evacuated interior region extending along the electron beam axis. Typically, the inner surface of the capsule is lined with an electrical insulator. Although the vacuum is used extensively for the insulation of high voltages in devices such as the x-ray probes described above, the reliability of the vacuum is limited by the operational risk of an unpredictable xe2x80x9csparkingxe2x80x9d or xe2x80x9carcingxe2x80x9d between the electrodes, when the insulating capability of the vacuum gap is suddenly lost and electrical breakdown is said to have occurred. Also, the efficient production of x-rays requires that the electron path be directly from the cathode to the target. If the electrons are deflected to the walls by effects of insulator charging, the efficiency of x-ray production is reduced, and stability of the x-ray output is compromised.
It is therefore important to establish a substantially uniform voltage gradient in the region between the electron source and the target, in order to avoid such electrical breakdown and to maximize and stabilize the x-ray output. It is an object of this invention to provide a high efficiency, miniaturized therapeutic radiation source having a substantially uniform voltage gradient within the vacuum region between the electron source and the target.
The present invention is directed to a high efficiency, miniaturized, optically driven therapeutic radiation source. The therapeutic radiation source includes an electron source and a target element that generates therapeutic radiation in response to incident accelerated electrons from the electron source. The electron source and the target element are enclosed within an evacuated capsule, whose inner surface is coated with a weakly conductive or semiconductive coating. In this way, chances of flashover and electrical breakdown in the evacuated capsule are substantially reduced, and the electrons are propagated directly from the cathode to the target.
The present invention features a therapeutic radiation source, which includes an optical source, a probe assembly, and a radiation generator assembly. The optical source is preferably a laser, or a light emitting diode. The probe assembly includes an optical delivery structure, such as a fiber optic cable, having a proximal end and a distal end. The fiber optic cable is adapted to transmit optical radiation incident on its proximal end to its distal end, and to direct a beam of optical radiation transmitted therethrough to impinge upon a surface of the thermionic cathode. This beam of optical radiation has a power level sufficient to heat at least a portion of the surface to an electron emitting temperature, so as to cause thermionic emission of electrons from the surface.
The radiation generator assembly includes an electron source, and a target element. The electron source includes a thermionic cathode having an electron emissive surface. The electron source is responsive to optical radiation transmitted to the distal end of the fiber optic cable, to generate an electron beam along a beam path along a nominally straight reference axis. The target element is positioned in the electron beam path. The target element includes at least one x-ray emissive material adapted to emit x-rays in response to incident accelerated electrons from the electron source.
A substantially rigid capsule encloses the electron source and the target element. The capsule defines a substantially evacuated interior region extending along the nominally straight beam axis, between the thermionic cathode at the proximal end of the capsule and a target element at the distal end of the capsule. The total resistance of the inner surface of the capsule is preferably high enough to limit dissipated power to less than 10% total target power.
The invention includes means for providing an accelerating voltage between the electron source and the target element, so as to establish an accelerating electric field which acts to accelerate electrons emitted from the electron source towards the target element. The accelerating voltage has a preselected maximum value.
The inner surface of the evacuated capsule is coated with a weakly conductive or semiconductive coating to provide a substantially smooth voltage gradient within the capsule, between the preselected maximum value and the ground potential. The weakly conductive or semiconductive coating, applied to the inner surface of the capsule, is also adapted to reduce secondary emissions of electrons striking the coated inner surface of the capsule. The weakly conductive or semiconductive coating is further adapted to reduce the electrical field in the vicinity of the triple junction point, thus reducing the possibility of electrical flashover the triple junction point of the thermionic cathode. Sufficient current is carried in the coating to prevent charge buildup from field emission, and subsequent avalanche and breakdown.